Two methods were performed to prevent the high effluent COD caused by lysis of filamentous bacteria when the nonionic surfactant was used to solve bulking problems. One method was repeated dose of low concentrations of the nonionic surfactant directly into the aeration tank. It could lower the effluent COD. However, the settleability of the bulking sludge was not improved when the concentration of the surfactant was too low. The other method was to lyse filamentous bacteria in a reactor, into which a part of the activated sludge was introduced from the aeration tank. In this method, the effluent COD was not raised by lysis of filamentous bacteria and the settleability of the bulking sludge was improved.